Field of the Invention
The invention relates to color displays, such as liquid crystal displays (LCDs), which convert electrical signals into color images. In particular, the invention concerns color, transmissive displays in which photoluminescence materials are used to generate color light in response to excitation radiation from a backlight, such displays being termed photoluminescence color displays or photoluminescent color displays.
Description of the Related Art
Light that allows us to see comes from solar energy in what is known as the visible region of the solar, electromagnetic, spectrum. This region is a very narrow segment of the total spectrum, the visible region being that portion visible to the human eye. It ranges in wavelength from about 440 nm in the extreme blue or near ultraviolet to about 690 nm in the red or near infrared. The middle of the visible region is a green color at about 555 nm. Human vision is such that what appears as white light is really composed of weighted amounts of a continuum of so-called black body radiation. In order to produce light that appears “white” to a human observer, the light needs to have component weights of about 30 percent in the red (R), 59 percent in the green (G) and 11 percent in the blue (B).
The perception of light as being white can be maintained even when the amount of one of the RGB component colors is changed, as long as the amounts of the other two can be adjusted to compensate. For example, if the red light source is shifted to a longer wavelength, the white light will appear more cyan in color if the other two colors remain unchanged. White balance may be restored, however, by changing the weight of the green and blue to levels other than their original values of 11 and 59 percent, respectively. The human eye does not have the ability to resolve closely spaced colors into the individual red, green, and blue (RGB) primary components of white light, since the human vision system mixes these three components to form intermediates. The reader will recall that human vision registers (and/or detects) only the three primary colors, and all other colors are perceived as combinations of these primaries.
Color liquid crystal displays (LCDs) in use today are based on picture elements, or “pixels,” formed by a matrix/array of liquid crystal (LC) cells. As is known, the intensity of the light passing through a LC can be controlled by changing the angle of polarization of the light in response to an electrical field, voltage, applied across the LC. For a color LCD, each pixel is actually composed of three “sub-pixels”: one red (R), one green (G), and one blue (B). Taken together, this sub-pixel triplet makes up what is referred to as a single pixel (pixel unit). What the human eye perceives as a single white pixel is actually a triplet of RGB sub-pixels with weighted intensities such that each of the three sub-pixels appears to have the same brightness. Likewise, when the human eye sees a solid white line, what is actually being displayed is a series or line of RGB triplets. The multi-sub-pixel arrangement may be manipulated by tuning the photometric output of the light source to a set of desired color coordinates, thereby offering a superior Color Rendering Index (CRI) and a dynamic color selection for a large color palette.
In current color, transmissive LCD technology, this color tuning is implemented with the use of color filters. The principle of operation of a conventional color, transmissive LCD is based upon a bright white light backlighting source located behind a liquid crystal (LC) matrix, and a panel of color filters positioned on an opposite side of the liquid crystal matrix. The liquid crystal matrix is digitally switched to adjust the intensity of the white light from the backlighting source reaching each of the color filters of each pixel, thereby controlling the amount of colored light transmitted by the RGB sub-pixels. Light exiting the color filters generates the color image.
A typical LCD structure is sandwich-like in which the liquid crystal is provided between two glass panels; one glass panel containing the switching elements that control the voltage being applied across electrodes of the LC corresponding to respective sub-pixel, and the other glass panel containing the color filters. The switching elements for controlling the LC matrix which are located on the back of the structure, that is facing the backlighting source; typically comprise an array of thin film transistors (TFTs) in which a respective TFT is provided for each sub-pixel. The color filter glass panel is a glass plate with a set of primary (red, green, and blue) color filters grouped together. Light exits the color filter glass panel to form the image.
As is known, LCs have the property of rotating the plane of polarization of light as a function of the applied electric field, voltage. Through the use of polarizing filters and by controlling the degree of rotation of the polarization of the light as a function of the voltage applied across the LC the amount of white light supplied by the backlighting source to the filters is controlled for each red, green and blue sub-pixel. The light transmitted through the filters generates a range of colors for producing images that viewers see on a TV screen or computer monitor.
Typically, the white light source used for backlighting comprises a mercury-filled cold cathode fluorescent lamp (CCFL). CCFL tubes are typically glass, and filled with inert gases. The gases ionize when a voltage is applied across electrodes positioned within the tube, and the ionized gas produces ultraviolet (UV) light. In turn, the UV light excites one or more phosphors coated on the inside of the glass tube, generating visible light. Reflectors redirect the visible light to the monitor and spread it as uniformly as possible, backlighting the thin, flat LCD. The backlight itself has always defined the color temperature and color space available, which has typically been approximately 75 percent of NTSC (National Television Standards Committee) requirements.
In the known LCD systems, the color filter is a key component for sharpening the color of the LCD. The color filter of a thin film transistor liquid crystal display (TFT LCD) consists of three primary colors (RGB) which are included on a color filter plate. The structure of the color filter plate comprises a black (opaque) matrix and a resin film, the resin film containing three primary-color dyes or pigments. The elements of the color filter line up in one-to-one correspondence with the unit pixels on the TFT-arrayed glass plate. Since the sub-pixels in a unit pixel are too small to be distinguished independently, the RGB elements appear to the human eye as a mixture of the three colors. As a result, any color, with some qualifications, can be produced by mixing these three primary colors.
The development over recent years of high brightness light emitting diodes (LEDs) has made possible LED backlighting with an enhanced color spectrum and has been used to provide a wider range of spectral colors for displays. In addition, LED backlighting has allowed for a tuning of the white point, when allied with a feedback sensor, ensuring the display operates consistently to a pre-defined performance.
In these LED based backlighting systems, the light output from red, green and blue (RGB) LEDs is mixed in equal proportions to create white light. This approach, unfortunately, requires complex driving circuitry to properly control the intensities of the three different color LEDs since different circuitry is necessary because each of the LEDs demands different drive conditions.
An alternative approach has been to use a white emitting LED which comprises a single blue LED chip coated with a yellow fluorescent phosphor; the yellow phosphor absorbing a proportion of the blue light emitted by the blue LED, and then re-emitting that light (in a process known as down-conversion) as yellow light. By mixing the yellow light generated by the yellow phosphor with the blue light from the blue LED, white light over the entire visible spectrum could be produced. Alternatively, an ultraviolet LED can be coated with a red-green-blue phosphor to produce white light; in this case, the energy from the ultraviolet LED is substantially non-visible, and since it cannot contribute a component to the resultant white light, it functions only as an excitation source for the phosphors. Unfortunately, the white light product of such LEDs does not match well with the color filters used in current LCDs, and a significant amount of the backlight intensity is wasted.
U.S. Pat. No. 4,830,469 proposes a LCD which uses UV light to excite red, green and blue light emitting phosphor sub-pixels thereby eliminating the need for RGB color filters. Such LCDs are referred to as photoluminescence color LCDs. A mercury lamp emitting UV light of wavelength 360 nm to 370 nm is used as a backlight and the red, green and blue emitting phosphors are provided on a front substrate plate. The UV light after being modulated by the liquid crystal matrix is then incident on the phosphor sub-pixels of the front plate which emit red, green and blue light in response.
U.S. Pat. No. 6,844,903 teaches a color, transmissive LCD which supplies a uniform blue light of wavelength 460 nm to the back of the liquid crystal layer. The blue light, after being modulated by the liquid crystal layer, is then incident on the back surface of phosphor material located above the liquid crystal layer. A first phosphor material, when irradiated with the blue light, generates red light for the red pixel areas of the display, and a second phosphor material, when irradiated with the blue light, generates green light for the green pixel areas of the display. No phosphor material is deposited over the blue sub-pixel areas since blue light is provided from the backlight. A suitable diffuser (e.g. scattering powder) can be located at the blue sub-pixel areas so that the blue pixels match the viewing angle properties of the red and green pixels.
United States Patent Applications US 2006/0238103 and US 2006/0244367 teach photoluminescence color LCDs which respectively use UV light of wavelength 360 nm to 460 nm and a near blue-UV light of wavelength 390 nm to 410 nm to excite red, green and blue light emitting phosphor sub-pixels. The use of near blue-UV backlighting reduces deterioration of liquid crystals caused by UV light.
A further example of a photoluminescence color LCD is disclosed in Japanese patent application JP 2004094039.
U.S. Pat. No. 8,947,619 discloses a photoluminescence color display comprising a photoluminescence color-elements plate having red, green or blue quantum dots material corresponding to the pixel areas of the display; and a wavelength selective filter disposed between the color-elements plate and the excitation source. The wavelength selective filter prevents light generated within the pixel areas returning to the excitation source.
The present invention concerns photoluminescence color display, which utilizes photoluminescence materials, such as quantum dots, inorganic and organic phosphor materials, to generate the different colors of light of the sub-pixels. What is needed in the art is a color display that uses an RGB photoluminescence based color rendering scheme to sharpen the color and enhance the brightness of the image.